Street Ironwork - risk mitigation or hiding risk?

Nick Orman is our Principal Consultant in Pipe Networks

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It's election time here and so I thought I would write about an interest that I am told I share with Jeremy Corbyn! Yes, you guessed it - it is manhole covers. Though I suspect he is more interested in the aesthetics rather than the engineering.

How many times have you seen this in the road ahead of you. Why is there a patch of surfacing on the cover?

The usual reason this is done is because there is a step which is considered a risk to vehicles; either because the cover has settled in the frame, or the frame and the cover have settled. The risk is that vehicles may be damaged due to the dip in the same way as they would if they went over a pothole.

Why was this done?

In the picture on the right, you can see that the cover has settled in the frame and the frame has also settled. You can also see that the work was done some time ago, so it can hardly be considered to be temporary.

In the picture on the left, you can’t see what it is covering up, so will anyone know the risk? In this case I happen to know that the frame had settled. You can also see that it was quite recent (actually only a few minutes before I took the photograph). I also know that the cover was replaced or reseated within a couple of days.

What are the risks?

If the cover has settled in the frame, then the risk depends entirely on the design of the cover. In the UK we are probably the only country that use double triangular frames with the bearing surfaces at the corners of the triangles. Loads are therefore concentrated on these corners and the bearing surfaces can wear due to a combination of corrosion and abrasion (they are in a salt-laden and gritty environment). There are two possibilities of risk here:

  • If the bearing surfaces are angled in the form of a V-notch, then the surface is large, although a small amount of wear can lead to quite a large settlement of the cover, but the risk of the support failing entirely is quite low unless the settlement was extreme.
  • If the bearing surface is flat, then the amount of remaining metal supporting the cover may be considerably reduced and there is a significant risk of failure. If a vehicle does drive over an open void there is a significant risk of a serious, if not fatal, accident.

In fact, the cover on the right has a V-notch bearing surface so this is not significant.

If the frame itself has settled then this is most likely caused by failure of the bedding mortar or the supporting brickwork, though it is possible that there is a more serious problem with the structure of part of the manhole structure itself. Once the bedding has failed, the movement of the manhole cover assembly under traffic leads to regular impact load on the supporting brickwork which typically starts to fail. In older manholes it could cause wider damage to the structure below. Bedding mortar can fail either due to poor specification, poor workmanship, or sewer flooding through the cover (this is indeed the case with the picture on the left).

The step or pothole itself can lead to a risk of damage to vehicles, but may be a more significant risk to cyclists.

Which is the greater risk?

So I would ask the following questions:

Did anyone check any of this before they covered up the evidence – did they do a risk assessment?

Why are these temporary solutions so often left to become permanent solutions? The problem was spotted yet only the temporary solution was undertaken.

Created by potrace 1.16, written by Peter Selinger 2001-2019

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Created by potrace 1.16, written by Peter Selinger 2001-2019

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Nick Orman

Principal Consultant (Wastewater Networks)

Nick is a Principal Consultant at WRc with over 35 years' experience in the water sector. He is a Chartered Civil Engineer and Chartered Water and Environmental Manager. His specialist areas include sewer inspection technologies, sewer deterioration mechanisms, sewer collapse analysis, sewer hydraulic modelling and cost analysis of sewer flooding schemes. Nick was a major contributor to the SRM Sewer Risk Management website and is the Technical Lead on the revision of the SRM Sewer Renovation Design Guide. He has been involved in the drafting or updating of many of the guidance documents relating to sewerage, examples include Sewers for Adoption, the Civil Engineering Specification for the Water Industry and the Manual for Sewer Condition Classification.

2024-06-04 13:22:00